Why do I get the felling that this thread is just wishful thinking?
Have you actually thought about:
How on earth you are going to clean it? Gravel syphon on a very long stick? You won't even be able to reach the bottom at the front of the tank, let alone in the middle if it's 5' wide. At around 5800L, even a 10% water change will be close to 600L, which will take up a heck of a long time just to fill back up with a hose.
Why you actually need it that big? I can't think of many fish that would need a 4' deep tank. With that depth you will run into all sorts of problems, getting enough light to the bottom of the tank (especially if you want plants), reaching the bottom for maintenance, decorating etc. Are you going to be keeping huge fish that need the width of 5' to turn around?
Have you researched the day-to-day running costs of a tank that big? How much power will the pumps/lights/heater use? What happens if the fish get sick and you need to medicate them? How much dechlorinator will you need to use during water changes? Even the cost of feeding such a large number of fish would be quite high.
What about the logistics of building and housing the tank? Obviously it will have to be built on-site, but what about the stand? The water alone will weigh close to 6 tons, plus the tank, stand, gravel, etc. Are you 100% sure your floor will be able to take that kind of weight? Can you even get a 10'x5' sheet of glass through your house and into the room where you want to build the tank?
This is not the kind of tank you can pack up and take with you when you move. Are you settled in that house, jobs etc? If you do move you will most likely have to pull the tank down as there's not many people who would want to take on such a large aquarium.
Building a tank of this size is an absolutely massive undertaking, and should be very thoroughly thought thru before spending a cent. How big is the biggest tank you are keeping now (or have ever kept)? I remember reading an article written by someone who volunteered to look after a big tank in a mall, I think it was a hexagon about 4' deep and had a large pacu and a couple of other big fish. He went into great detail about all the problems of such a large tank, and said after looking after such a large tank his 'dream tank' went from 600g+ with massive fish to about 240g with fish that only grew to around 12".
Sometimes bigger isn't always better....